In a cordless telephone system, it is necessary to provide a way of carrying the signal for the contents of communication, normally speech, in both directions between parts of the system which are not connected by a cord or wire. Additionally, it will normally be necessary to pass other signals between the parts, which are used to control the operation of the parts or carry other control messages separate from the content of the communication. In some known radio telephone systems, the requirement for two way communication is achieved by providing two radio channels between the parts, each channel being used for communication in one respective direction. In an embodiment of the present invention, multiplexed signal structures are provided enabling a plurality of logical channels to be carried with communication in both directions, over a single signal communications channel. In the embodiment, the use of different multiplex structures at different times permits differences in the logical channel structure of the cordless communication at different stages of the creation and use of the cordless communications link.
In a conventional radio telephone system, an arrangement must be provided enabling a link to be set up so that parts can communicate with each other. When one of the parts is operating in a manner synchronised to some routine, it may be difficult for another part to establish a link with the first part if the second part is not itself synchronised to the same routine. In an embodiment of the present invention, an arrangement is provided allowing for asynchronous initiation of a link between two parts, even when one of the parts is operating in a synchronous manner.
In a radio telecommunication system, the ability of a radio link to carry useful signals will tend to vary in accordance with external factors, such as interference and transmission past obstructions. Accordingly, it is advantageous to encode transmitted signals for error detection and correction and/or monitor the link quality to enable remedial steps such as breaking and re-establishing the link, possibly on a different radio channel, if the link quality becomes unacceptably low. In an embodiment of the present invention, an arrangement is provided in which two logical channels are multiplexed together, with signals of one logical channel being encoded to enable error detection, and detected errors in this logical channel being monitored and used as a measure of the extent to which the other channel is exposed to errors.
In a system for radio telecommunications, there will typically be a large number of communication devices capable of communicating in the system, some of which may be more sophisticated and have greater communication abilities than others. In order for two devices to communicate with each other, they must communicate in a manner which is within the capabilities of both devices. Thus, when a relatively sophisticated device communicates with an unsophisticated device, they must communicate in a manner within the capabilities of the unsophisticated device. However, it is inefficient to force the sophisticated device also to communicate in this particular manner when it is communicating with another sophisticated device capable of communicating in a different manner. In an embodiment of the present invention, devices conduct an operation (sometimes referred to as a "negotiation" operation) during the creation of a cordless telecommunications link, so as to adopt a way of communicating which is within the capabilities of both devices.
When two devices are communicating over a cordless telecommunications link, it may be necessary for the operations of the devices to be synchronised with each other and this may be done by one device recognising a particular part of a signal transmitted by the other device, the signal part having a predetermined timing. In this case, incorrect synchronisation can arise if the receiving part incorrectly identifies a different part of the transmitted signal as the part to be recognised for synchronisation. In an embodiment of the present invention, a signal is transmitted having a data structure such that a portion used for synchronisation has a low correlation with other portions of the signal not containing the synchronisation part. Additionally, in the embodiment the synchronisation part has a low correlation with time-shifted versions of itself. Preferably, signal parts used for synchronisation are transmitted in both directions between the devices, and a signal part used for synchronisation and transmitted in one direction is arranged to have a low correlation with a signal part used for synchronisation and transmitted in the other direction.
When a plurality of devices capable of communicating over a cordless communications link are present in the same area, and several are scanning communication channels to detect another device seeking to set up a communications link, there is a possibility that two devices may detect the same request for a communications link on a channel and both respond to the request simultaneously. The resulting interference on the channel may result in neither device establishing the communications link. If the subsequent behaviour of the two devices in scanning the channels for requests for a communications link is identical, such a simultaneous response and interference is likely to occur with every subsequent detection of a communication request. In an embodiment of the present invention, some devices are arranged so as to have a behaviour following such a simultaneous response and interference which is different from each other, so as to reduce the likelihood of subsequent repetitions of the simultaneous response and interference.
Devices communicating with each other over a cordless telecommunications link may exchange "handshake" signals to confirm that communication between them over the link is still taking place successfully. If one of the devices fails to receive a handshake signal within a certain period, it may conclude that the link has been broken. However, the device which has ceased to receive handshake signals will typically still be transmitting them until the end of the period at which it concludes that the link has been broken. If these handshake signals are successfully received by the other device, then the other device will not become aware of the failure of the link until a further period after the first device ceases to transmit handshake signals. Thus, when a transmission link fails in one direction only, the reaction of the devices may be delayed and will typically not be synchronised with each other. In an embodiment of the present invention, if a device fails to receive a handshake signal within a first period of its most recent receipt of a handshake signal, it concludes that the link has been lost. In the meantime, it continues to transmit handshake signals, but if it has not received a handshake signal within a second, shorter, period since the most recent handshake signal, it transmits a signal indicating that it is failing to receive handshake signals. Thus, if a link breaks down in one direction only, the device which is continuing to receive transmissions is rapidly notified that the other device has ceased to receive transmissions, and the link re-establishment actions of the devices can be better co-ordinated.
Where two devices communicate with each other in a synchronised manner, it is possible for the transmission of information to become corrupted by a loss of synchronisation, even though the transmission quality of the communication link may be unimpaired. In an embodiment of the present invention, some of the transmitted information is coded to enable error detection, and the detection of errors in this data can be used as an indication that synchronisation between the devices has been lost.
When two devices are communicating with each other over a communications link, in a synchronised manner, one of them may be designated a synchronisation master, and the other a synchronisation slave, such that the slave is required to synchronise itself to the operations of the master. If the link fails, or , for any other reason the devices are required to break and re-establish the link, re-establishment may be difficult if the slave device ceases to be synchronised to the master and therefore fails to detect link re-establishment signals from the master. In an embodiment of the present invention, when link re-establishment is required the initial signalling to carry out the re-establishment is always transmitted by the slave device.
If one of the devices in a link is portable or mobile, the link may be broken by the movement of that device. It may then be impossible to re-establish the link between the same two devices. If one of the devices is also an endpoint of a communications path, e.g. a handset, and the other is only a relay station, e.g. a base station linked to a communications network, it is preferable to re-establish the link using the same endpoint device, but possibly a different relay device, for the convenience of the user. However, it may be difficult for the communications network to monitor which relay device an endpoint device is near at any given time. In an embodiment of the present invention the endpoint device transmits the initial signals in link re-establishment, and the link can be re-established using any relay device which receives the transmissions. The endpoint device is typically the mobile device, e.g. a portable telephone handset.
When devices are communicating over a communications link using an alternating transmission burst arrangement, there may be a failure of communication if the timings of the transmissions of the two devices are not properly co-ordinated and their transmissions partially overlap instead of alternating correctly. In an embodiment of the present invention one device derives the timing for the transmission of a burst from the time at which it receives a burst from the other device.
When devices communicate over a telecommunications link, it may be necessary to transmit signals belonging to a logical channel for the purposes of link maintenance, even though there is no information to be transmitted in that channel at that time. If a random signal is sent in that logical channel under the circumstances, it may by chance resemble some meaningful signal transmitted over the communications link, resulting in incorrect operation of the device receiving the signal. In an embodiment of the present invention, a specified signal structure is provided for a logical channel which structure conveys no useful information in that channel, but which is chosen not to resemble a signal the reception of which could cause incorrect operation of the receiving device.